This invention relates to a transmission rate control system that supports dynamic bandwidth allocation for a circuit emulation service in a connection-oriented packet switched network, such as ATM (Asynchronous Transfer Mode). This mode of operation is described in the ATM Forum specification number af-vtoa-0085.000.
For convenience the invention will be described with reference to ATM, which is a global standard for a cell-relay, connection-oriented network, although persons skilled in the art will recognize that the invention is applicable to other packet switched networks.
In ATM, an incoming data stream is segmented into 53 byte cells, which have five bytes of header and 48 bytes of payload. The ATM cells from different sources are statistically multiplexed over physical links to create virtual channels between endpoints. ATM offers various types of service, of which AAL1, which stands for ATM Adaptation Layer 1, is of interest for the present application.
AAL1 service is a constant bit rate (CBR) service, suitable for carrying voice traffic. AAL1 appears to the user as a circuit connection that permits the setting up of telephone calls in a manner similar to a conventional telephone circuit. For this reason, it is known as a circuit emulation service (CES) because virtual circuits can carry multiple telephone channels in a manner that to the user appears analogous to physical circuits.
Typically, multiple telephone calls are carried on trunks, which may in reality be time division multiplexed trunks, such as T1 trunks, which carry 24 simultaneous calls in 24 time slots per frame. Each time slot carries 8 bits and the frame repeat at 8 KHz, giving a transmission rate per channel of 64 kb/s.
More and more users are setting up ATM virtual circuits to from wide area networks, and such users are desirous of using these circuits to carry voice data. In order to convert incoming voice channels into a suitable form for transmission over these virtual circuits, the incoming data must be assembled into cells, which must then be transmitted at the appropriate time and disassembled at the far end. A device for performing this function is known as a SAR (Segmentation and Reassembly) device.
To provide a constant bit rate service, some means must be found to determine when to produce cells for transmission. Historically, a scheduler mechanism has been used in which TDM basic frames are counted and cells are produced after a fixed number of TDM frames have occurred.
In a constant bit rate service, for example, carrying voice, there will be periods of silence when neither party is talking. Since it is wasteful of ATM bandwidth to transmit silence, the Dynamic Bandwidth Circuit Emulation Service (DBCES) was introduced. This dynamically allocates available ATM bandwidth to incoming TDM channels. The scheduler system is unsuitable for DBCES applications due to the variable number of TDM frames between cells depending on the amount of silence in the TDM voice channels.
An object of the invention is to provide a cell rate transmission control system (CTRCS) compatible with DBCES operations.
According to the present invention there is provided a cell transmission rate controller for use in providing constant bit rate services in a cell relay network, comprising a predictor for determining the payload size of the next cell in a virtual circuit established in said cell relay network, and a control circuit for monitoring buffers storing incoming TDM data and generating a trigger signal to produce a cell when sufficient TDM data has accumulated to fill the next cell.
A Cell Transmission Rate Control System (CTRCS) in accordance with the invention is used in the Segmentation Sub-layer of an ATM Adaptation Layer type 1 (AAL1). It synchronizes ATM cell production to an isochronous TDM link rate. The CTRCS allows the SAR to support Circuit Emulation Service (CES) as described in the ATM Forum Specification, af-vtoa-0078.00. In addition the CTRCS may be used in SAR implementations which support Dynamic Structure Sizing (DSS) as described in the ATM Forum Specification, af-vtoa-0085.000.
The CTRCS determines the number of bytes or octets in the next cell which are available to transport TDM information. The CTRCS monitors a number of circular buffers (one for every channel in the VC) which are being filled with TDM information. When enough data has accumulated to fill the next cell the CTRCS will trigger the Segmentation System to produce a cell. A single CTRCS may be used with multiple virtual circuits and multiple links. There is no limit to the number of channels which may be carried by the VC and the CTRCS may be configured to support E1/DS1/J2 Nxc3x9764 AAL1 structures with Common Channel Signaling (CCS) and Channel Associated Signaling (CAS).
The CTRCS thus serves a replacement for the scheduler in DBCES applications. A further advantage of the CTRCS is that whereas a separate scheduler is required for each TDM link which is supported by the SAR, a single CTRCS is capable of supporting any number of TDM links and an unlimited number of virtual circuits (VCs).